In Flutter, when you declare variables in different contexts like top-level, within a State class, or inside the build method of a stateful widget, they serve different purposes and have different scopes. Let's explore each of them:

Top-level variables are declared outside of any class or function, at the top of the file. These variables have a global scope and can be accessed and modified from anywhere in the same file. However, they are not associated with any specific widget and are generally used for values or constants that need to be shared across multiple widgets.

Understanding Top-Level Variables in more detail:

i. As mentioned before, top-level variables are declared outside of any class or function in a Dart file.

ii. They have a global scope, meaning they can be accessed and modified from anywhere within the same file.

iii. Top-level variables are initialized only once when the application starts, and their values remain consistent throughout the app's runtime.

iv. These variables are typically used for constants, configuration values, or data that need to be shared across multiple widgets or classes.

const String apiUrl = "https://api.example.com";

When you create a stateful widget in Flutter, you typically define a corresponding State class for that widget. Within the State class, you can declare instance variables that hold the widget's state. These variables are associated with the particular instance of the widget and can be modified during the widget's lifetime. When the state of the widget changes, Flutter will automatically rebuild the widget with the updated values.

Understanding State Class Variables in more detail:

i. State class variables are declared inside the State class of a stateful widget.

ii. They are associated with a particular instance of the widget and are responsible for managing the widget's state.

iii. When the state of a stateful widget changes (due to user interactions or other events), the corresponding state class variables are updated, triggering a rebuild of the widget's UI.

iv. Since stateful widgets can be rebuilt multiple times during their lifetime, the state class variables preserve the widget's state across rebuilds.

class CounterWidget extends StatefulWidget {

  _CounterWidgetState createState() => _CounterWidgetState();

class _CounterWidgetState extends State<CounterWidget> {

        ElevatedButton(onPressed: _increment, child: Text('Increment')),

The build method is a required method in a stateful widget that returns the widget's UI representation. In the build method, you can declare local variables that are used to build the UI or perform computations specific to the widget's appearance.

Understanding Build Method Variables in more detail:

i. The build method is responsible for creating the widget's UI representation based on its current state and properties.

ii. Within the build method, you can declare local variables that are used to build the UI or perform temporary computations that are specific to that particular render of the widget.

iii. Unlike state class variables, the variables declared inside the build method do not survive widget rebuilds. Each time the build method is called, the local variables are reinitialized.

Here's an Build Method Variables example:

class WeatherWidget extends StatelessWidget {

  WeatherWidget({required this.cityName});

    // Local variable in the build method

    String weatherStatus = getWeatherStatus(cityName);

      child: Text('Weather in $cityName: $weatherStatus'),

  String getWeatherStatus(String cityName) {

    // Simulate getting weather status from an API or other logic

    // This function is not in the build method to prevent unnecessary recomputation.

Remember, while local variables inside the build method can be helpful for computations and UI building within the widget, you should avoid heavy computations in the build method itself. Intensive computations might cause performance issues since the build method can be called frequently during the widget's lifecycle. It's better to perform such computations outside the build method and store the result in state class variables or top-level variables when needed.

In conclusion, understanding the different variable scopes in Flutter is essential for effective state management and UI building. Top-level variables have global access, state class variables manage widget state, and build method variables are for local UI-related computations. Each serves a specific purpose and has its own lifetime. Properly using these scopes can lead to well-organized and performant Flutter applications.

In this article, we will discuss Flutter variable scopes which are Top-level (global), State class (widget state), and Build method (local UI-related) different purposes and lifetimes.

Flutter Variables Scopes

 How can i save the value in the Counter? If i start my Code, then the button is working, but it will reset to zero, so if i click the "+" Button, so it is always by 1. And when i click the "-" Button, then the count is by "-1". It does not go upper or lower. And every 24h i want a reset to zero. Can someone help me? 

import android.app.PendingIntent;
import android.appwidget.AppWidgetManager;
import android.appwidget.AppWidgetProvider;
import android.content.ComponentName;
import android.content.Context;
import android.content.Intent;
import android.widget.RemoteViews;

import java.lang.reflect.Type;

public class Water extends AppWidgetProvider {

    private static final String MyOnClickPlus = "Plus";
    private static final String MyOnClickMinus = "Minus";
    int sum = 0;

    @Override
    public void onUpdate(Context context, AppWidgetManager appWidgetManager, int[] appWidgetIds) {
        RemoteViews views = new RemoteViews(context.getPackageName(), R.layout.water);
        views.setOnClickPendingIntent(R.id.btnplus, getPendingSelfIntent(context, MyOnClickPlus));
        views.setOnClickPendingIntent(R.id.btnminus, getPendingSelfIntent(context, MyOnClickMinus));


        appWidgetManager.updateAppWidget(appWidgetIds, views);
        super.onUpdate(context, appWidgetManager, appWidgetIds);
    }

    @Override
    public void onReceive(Context context, Intent intent) {
        // TODO Auto-generated method stub
        RemoteViews views = new RemoteViews(context.getPackageName(), R.layout.water);

        if(intent.getAction().toString() == MyOnClickPlus){
            sum += 1;
            views.setTextViewText(R.id.mengeheute, Integer.toString(sum));
        }
        else if(intent.getAction().toString() == MyOnClickMinus) {
            sum -= 1;
            views.setTextViewText(R.id.mengeheute, Integer.toString(sum));
        }
        updateWidget(context, views);
        super.onReceive(context, intent);
    }

    protected static PendingIntent getPendingSelfIntent(Context context, String action) {
        Intent intent = new Intent(context, Water.class);
        intent.setAction(action);
        return PendingIntent.getBroadcast(context, 0, intent, 0);
    }

    void updateWidget(Context context, RemoteViews remoteViews){
        ComponentName widgetComponent = new ComponentName(context, Water.class);
        AppWidgetManager.getInstance(context).updateAppWidget(widgetComponent, remoteViews);
    }
 

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.water">

    <application
        android:allowBackup="true"
        android:icon="@drawable/drinkwater"
        android:label="@string/app_name"
        android:roundIcon="@drawable/drinkwater"
        android:supportsRtl="true"
        android:theme="@style/Theme.Water">

        <activity
            android:name=".MainActivity"
            android:exported="true">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <receiver
            android:name=".Water"
            android:exported="true">
            <intent-filter>
                <action android:name="android.appwidget.action.APPWIDGET_UPDATE" />

            </intent-filter>

            <meta-data
                android:name="android.appwidget.provider"
                android:resource="@xml/water_info" />
        </receiver>
    </application>

</manifest>

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